Inorganic glass has been widely used in various fields as a transparent optical material, because of its favorable properties, e.g., high transparency and low optical anisotropy. However, it has some disadvantages of being heavy, fragile, poor in productivity and the like. Therefore, various optical resins have been extensively developed to replace the inorganic glass.
One of the basically most important characteristics as the optical resins is transparency. The highly transparent optical resins which are known so far include polymethyl methacrylate (PMMA), bisphenol A polycarbonate (BPA-PC), polystyrene (PS), methyl methacrylate/styrene copolymer (MS), styrene/acrylonitrile copolymer (SAN), poly(4-methyl-1-pentene) (TPX), polycycloolefin (COP), polydiethylene glycol bisallyl carbonate (EGAC) and polyurethane (PTU).
The PMMA has been widely used as one of the typical optical resins, because of excellent transparency and weather resistance as well as good moldability. However, it has disadvantages of a relatively low refractive index (nd) of 1.49 and high water absorbing properties.
The BPA-PC has been used in optical applications typified by optical disk boards for information record, because of excellent transparency, heat resistance and impact resistance as well as a relatively high refractive index (nd: 1.59). However, the BPA-PC has disadvantages of a relatively high aberration (dispersion of refractive index) and birefringence, high melt viscosity, and relatively poor moldability, which have limited its applications as the optical resin.
The PS and MS have high moldability, high transparency, low water absorbing properties and a high refractive index. However, they have been scarcely used as the optical resins, because of their disadvantages of insufficient resistance to impact, weather and heat. Furthermore, the SAN is relatively high in refractive index, and is considered to have balanced mechanical properties. However, it is also scarcely used as the optical resin, because of its relatively insufficient heat resistance (thermal deformation temperature: 80 to 90° C.).
The TPX and COP, although being excellent in transparency, having low water absorbing properties and being excellent in heat resistance, have disadvantages of a low refractive index (nd: 1.47 to 1.53), and insufficient impact resistance, gas barrier properties and dye-affinity.
The EGAC is a thermosetting resin obtained by polymerizing diethylene glycol bisallyl carbonate which is a monomer, and has been most widely used for common spectacles lenses. It has favorable characteristics of high transparency, high heat resistance and a very low chromatic aberration, but disadvantages of a low refractive index (nd: 1.50) and slightly poor impact resistance.
The PTU is a thermosetting resin obtained by the reaction between a diisocyanate compound and a polythiol compound, and most widely used at present for spectacles lenses of a high refractive index. It is the very excellent optical resin having, particularly, excellent transparency and impact resistance, and a high refractive index and a relatively low chromatic aberration. However, it needs a long thermal polymerization/molding time (1 to 3 days) in a spectacles lens production process, which is essentially its sole major disadvantage. Accordingly, it involves the productivity-related problems.
Novel polycarbonate-based thermoplastic optical resins have been proposed, in order to solve the disadvantages involved in the above-described bisphenol A polycarbonate (BPA-PC, hereinafter referred to as the general-purpose polycarbonate) which is one of the typical optical resins, and to produce high-quality optical components in a short time by injection molding. For example, Japanese Patent Laid-open Publication Nos. 66234/1989 and 223119/1989 disclose that a polymer such as an alicyclic polycarbonate copolymer having a repeating structural unit derived from an alicyclic dihydroxy compound has a relatively low chromatic aberration (high Abbe number) and a low birefringence, and they propose its optical applications. The polymers given by these processes can be injection-molded in a short time into optical components. However, it is difficult for these components to sufficiently satisfy requirements of the practical optical components, although having favorable characteristics, e.g., a high Abbe number or a relatively low birefringence. More specifically, they have several practical problems to be solved when used for spectacles lenses, and for example, they are relatively low in refractive index and hardly sufficient in heat resistance.
As described above, each of the conventional optical resins involves its own disadvantages and problems to be solved, though has been put to practical use while taking its characteristics into consideration for specific purposes. Under such circumstances, there are nowadays-keen demands for novel thermoplastic optical resins which are excellent in transparency and optical characteristics (e.g., a high refractive index, a high Abbe number and a low birefringence), good in mechanical characteristics (e.g., impact resistance) and thermal characteristics (e.g., thermal deformation temperature), and excellent in melt fluidity.